In general, a display apparatus equipped with a color display tube (color picture tube) is provided with a brightness or intensity control circuit for adjustably changing the brightness of a raster scanned on a face plate of the color display tube. Such a brightness control circuit has to be so designed that only the brightness of the raster produced through beam scanning on the face plate can be varied without bringing about any change in the color tone of the raster. Further, the brightness control circuit must be so arranged as to change only the brightness of the raster when the latter is at a low or high level.
Usually, the raster on the face plate of the color display tube is produced by scanning with three electron beams an array of triplets of phosphor dots for red, green and blue sequentially disposed on the face plate of the color display tube. The three electron beams are emitted by three cathode electrodes mounted within the color display tube. The brightness of the raster undergoes variation when the quantity of electron beams, i.e. the currents, is changed. These currents in turn vary when the voltages applied to the cathodes of the color display tube undergo variation. In other words, change in the voltages applied to the cathodes brings about a corresponding change or variation in the brightness of the raster. When the three cathode voltages are changed independent of one another, the amounts of light in red, green and blue produced by the phosphor dots also vary independent of one another. The result of this is that variations occur not only in the brightness of the raster but also in the color tone thereof. When the three cathode voltages are changed with the voltage ratios thereamong being selected at appropriate values, only the brightness can be changed without exerting influence on the color tone. The circuit for selecting the cathode voltages of the color display tube at appropriate values when the brightness of the raster is low is known as the cut-off voltage control or regulating circuit, a typical one of which is disclosed in Japanese Patent Application Laid-Open No. 138987/1980. In FIG. 1 of the accompanying drawings, there is shown this cut-off voltage control or regulating circuit. Referring to the Figure, a color display tube 100 includes three cathode electrodes 111, 112 and 113 which are connected to a power source terminal 1 (V.sub.cc') by way of resistors 161, 162 and 163, respectively. Further, the cathodes 111, 112 and 113 are, respectively, connected to cut-off voltage control or regulating circuits 140, 150, and 160 by way of terminals 171, 172 and 173. The other terminals 174, 175 and 176 of the cut-off voltage regulating circuits 140, 150 and 160 are connected to a color display tube drive circuit 180 through terminals 177, 178 and 179. The color display tube drive circuit 180 includes transistors 181, 182 and 183 having collectors connected to the terminals 177, 178 and 179, respectively. Bases of the transistors 181, 182 and 183 are, respectively, connected to terminals 184, 185 and 186 to which video signals are supplied. The video signals applied to the terminals 184, 185 and 186 are amplified by the transistors 181, 182 and 183 of the color display tube drive circuit 180 and supplied to the cathodes 111, 112 and 113 of the color display tube 100 from the terminals 177, 178 and 179 of the color display tube drive circuit 180 by way of the cut-off voltage control or regulating circuits 140, 150 and 160, respectively. Connected to the collectors of the transistors 181, 182 and 183 are resistors 187, 188 and 189 which in turn are connected to a power source terminal 2 (V.sub.cc). The voltage V.sub.cc at the power source terminal 2 is set at a lower level than the voltage V.sub.cc' of the power source terminal 1. Unless the video signal is supplied to the input terminals 184, 185 and 186 of the color display tube drive circuit 180, the transistors 181, 182 and 183 remain in the cut-off state. Accordingly, a DC current flows from the power source terminal 1 to the power source terminal 2 through the resistors 161, 162 and 163, the cutoff voltage control or regulating circuits 140, 150 and 160 and the resistors 187, 188 and 189. The cut-off voltage control or regulating circuit 140 includes a variable resistor 141, a transistor 142, a constant-voltage diode (avalanche diode) 143 and a capacitor 144. The cut-off voltage control or regulating circuits 150 and 160 also are constituted by components similar to those of the cut-off voltage control or regulating circuit 140. The resistance values of the resistors 187, 188 and 189 are selected to be so small that voltages E.sub.1R, E.sub.1G and E.sub.1B appearing at the terminals 174, 175 and 176 may be substantially equal to the voltage V.sub.cc of the power source terminal 2.
Therefore, the voltages E.sub.2R, E.sub.2G and E.sub.2B at the terminals 171, 172 and 173 respectively are lower than the voltage V.sub.cc' at the power source terminal 1. Voltages (E.sub.2R -E.sub.1R), (E.sub.2G -E.sub.1G) and (E.sub.2B -E.sub.1B) appearing between the terminals 171, 172 and 173 and the terminals 174, 175 and 176 are, respectively, changed by the cut-off voltage control or regulating circuits 140, 150 and 160. By way of example, in the cut-off voltage control or regulating circuit 140, the voltage (E.sub.2R -E.sub.1R) is changed or regulated by means of the variable resistor 141. Functions of the cut-off voltage control circuits 140, 150 and 160 will be described hereinafter. Since the voltages E.sub.1R, E.sub.1G and E.sub.1B are substantially equal to the voltage V.sub.cc, the voltages E.sub.2R, E.sub.2G and E.sub.2B will change when the voltages (E.sub.2R -E.sub.1R), (E.sub.2G -E.sub.1G) and (E.sub.2B -E.sub.1B) are changed. When the voltages E.sub.2R, E.sub.2G and E.sub.2B at the terminals 171, 172 and 173 are increased, the voltages applied to the cathodes 111, 112 and 113 of the color display tube 100 are also increased. On the other hand, when the voltages E.sub.2R, E.sub.2G and E.sub.2B are lowered, the voltages of the cathodes 111, 112 and 113 become lower, which results in that the potential differences between the cathodes 111, 112 and 113 and a grid G.sub.1 are decreased. Consequently, the quantity or amount of the electron beams emitted by the cathodes 111, 112 and 113 and passing through the grid G.sub.1 is increased. When the voltages of the cathodes 111, 112 and 113 become higher, the amount of the electron beams passing through the grid G.sub.1 is decreased. If the voltages of the cathodes 111, 112 and 113 are further increased, the electron beams are inhibited from passing through the grid G.sub.1 and thus are cut off by the grid G.sub.1. Usually, the voltages E.sub.2R, E.sub.2G and E.sub.2B of the terminals 171, 172 and 173 are so selected that they are very close to the voltages at which the electron beams emitted from the cathodes 111, 112 and 113 are cut off by the grid G.sub.1 and that the correct white balance can be obtained. When the video signals are supplied to the input terminals 184, 185 and 186 of the drive circuit 180, the transistors 181, 182 and 183 are turned on (i.e. changed over to the conducting state), whereby the voltages at the output terminals 177, 178 and 179 of the drive circuit 180 are lowered, which results in that the voltages. E.sub.1R, E.sub.1G and E.sub.1B at the terminals 174, 175 and 176 of the cut-off voltage control or regulating circuits 140, 150 and 160 are lowered. Thus, the voltages E.sub.1R, E.sub.2G and E.sub.2B at the terminals 171, 172 and 173 are lowered, to thereby cause the voltages of the cathodes 111, 112 and 113 of the color display tube 100 to be lowered. The electron beams emitted from the cathodes 111, 112 and 113 can pass through the grid G.sub.1, whereby a raster is produced on the face plate 120 of the color display tube 100. By the way, the video signal applied to the terminal 174 of the cut-off voltage control circuit 140 is supplied to the terminal 171 through the capacitor 144.
The cut-off voltage control or regulating circuits 141, 142 and 143 shown in FIG. 1 can change the amount of the electron beams emitted by the cathodes 111, 112 and 113 of the color display tube 100 without undergoing any appreciable influence of the operation of the drive circuit 180 or exerting any appreciable influence to the operation or function of the drive circuit 180. Accordingly, by selecting the voltages at the terminals 171, 172 and 173 of the cut-off voltage control circuits 140, 150 and 160 at appropriate values and varying the latter, only the brightness will be changed without bringing about any change in the color tone or chromacity of the raster. For this reason, it is possible to constitute the brightness control circuit with the aid of the cut-off voltage control circuits 140, 150 and 160.
Usually, the brightness control means of the color display apparatus is mounted on a housing thereof (not shown), while the drive circuit 180 is disposed at a location distanced from the color display tube 100. By way of example, assuming that the resistor 141 is disposed on the housing of the color display apparatus, e.g. on a front panel of the housing, the length of each of lines 145 and 146 which serve to connect the resistor 141 to the terminals 171 and 174 will amount to about 30 cm. As the consequence, stray capacity will be produced between the common potential (ground potential) and the lines 145 and 146. In particular, when shielding wires are used for the lines 145 and 146 with a view to preventing the spurious emission from the lines 145 and 146, the stray capacity will amount to 50 pF or more. In case the variable resistor 141 is disposed on a same base plate or substrate as the cut-off voltage control or regulating circuit 140 in the apparatus shown in FIG. 1 to thereby allow the lines 145 and 146 to be extremely short, the stray capacity produced by the color display tube 100, and the cut-off voltage control or regulating circuit 140 is only about 5 pF. Accordingly, if the lines 145 and 146 providing the stray capacity of about 50 pF are connected to the cut-off voltage regulating circuits 140, 150 and 160, the frequency passband of the circuit will be narrowed to about one-tenth. In this connection, it is noted that the circuit exhibiting the frequency pass-band beyond 50 MHz is demand in the modern color display apparatus. The circuit of the narrow frequency pass-band tends to be no more used.